The present invention relates to a color image pickup device such as a video camera.
In the color image pickup device such as a video camera, proper color reproduction cannot be performed unless a white balance is controlled at picture taking in accordance with the color temperature of the object to be photographed. Several methods have been proposed to control the white balance. FIG. 1 shows a typical system for controlling the white balance in a color video camera. Referring to FIG. 1, a lens 102 is focused on an object 101. Light from the object 101 passes through a trimming filter 103 through the lens 102, so that the image of an object is formed on an image pickup surface of a pickup tube 104. The pickup tube 104 comprises color stripe filters shown in FIG. 2. Reference symbol W denotes a transparent filter; G, a green filter; Ye, a yellow filter; and Cy, a cyan filter. The red and blue components of incident light are produced as an AM-modulated high-frequency signal from the stripe filters. This high-frequency signal is known as an interleave signal having a single carrier frequency. If the carrier frequency is 3.58 MHz, the interleave signal is amplified by a preamplifier 105 and is filtered by a band-pass filter 106 which has a frequency band of 3.58 MHz. The band-pass filter 106 then produces a chrominance signal. The chrominance signal is then delayed by a delay line 107 of (1H-.pi./2). The delayed chrominance signal is then added to and subtracted from a non-delayed chrominance signal. An output from an adder 108 is detected by a detector 109. The band of the detected signal is then limited by a low-pass filter (LPF1) 110 which has a frequency band of 500 kHz. The low-frequency signal is then amplified by a voltage controlled amplifier (VCA) 111 which thus produces a red signal R. Similarly, an output from a subtractor 112 is detected by a detector 113. The band of the detected signal is limited by a low-pass filter (LPF1) 114, and the low-frequency signal from the low-pass filter 114 is then amplified by a voltage-controlled amplifier (VCA) 115 which produces a blue signal B. The voltage controlled amplifier is an amplifier which controls a gain in accordance with a voltage applied to the control end thereof. Meanwhile, the output from the preamplifier 105 is supplied to a low-pass filter (LPF1) 116 and a delay line (117), so that the output from the preamplifier 105 is converted into a low-frequency luminance signal Y.sub.L of the same frequency band and synchronous with the red and blue signals R and B. At the same time, the output from the preamplifier 105 passes through a low-pass filter (LPF2) 118 of 3.1 MHz and a delay line 119 and is supplied to an NTSC encoder 120. The NTSC encoder 120 produces a luminance signal Y which is synchronized with the red, blue and low-frequency luminance signals R, B and Y.sub.L. The signals R, B and Y.sub.L are encoded by the NTSC encoder 120 and are produced as an NTSC signal 121.
Assume that the video camera is arranged such that the spectral characteristics of the pickup tube and the stripe filters are designed so as to provide a good balance of the color signal output under an illumination which has a color temperature of 3,000.degree. K., as shown in FIG. 3A. When the user operates such a video camera under an illumination of 6,000.degree. K. or in outdoors under direct sunlight, the output balance of the signals B, Y.sub.L and R changes, as shown in FIG. 3B. In order to balance the spectral sensitivities of the signals B, Y.sub.L and R, an amber filter as a trimming filter 103 is arranged on the optical path. Furthermore, a diffusion plate 122 is disposed on the optical path so as to uniformly illuminate the surface of the pickup tube with external light. An aperture of the lens is controlled such that the output from the pickup tube is kept normal, that is, a signal current is 0.2 .mu.A.sub.p-p when a vidicon tube is used. However, the aperture may be controlled by an automatic diaphragm mechanism. In this case, an output from the VCA 111 corresponds to the red signal which has a given level relative to the white signal when the user just turns on the power switch of the camera. The given level is determined by a gain in accordance with the initial status of a voltage memory. This red signal is detected by a detector 128 and is smoothed by a smoothing circuit (LPF3) 129, thereby obtaining a DC voltage. The DC voltage is applied to one input end of a differential amplifier 130. The differential amplifier 130 compares the DC voltage from the smoothing circuit 129 and another voltage applied thereto, and produces a signal corresponding to a potential difference. The signal from the differential amplifier 130 is stored in a voltage memory 132 through an analog switch 131 which performs color temperature control. The blue signal is also processed in the same manner as the red signal. More particularly, the blue signal passes through a detector 123, a smoothing circuit 124 and a differential amplifier 125. The signal from the differential amplifier 125 is stored in a voltage memory 127 through an analog switch 126. Meanwhile, the low-frequency luminance signal Y.sub.L is detected by a detector 133 and is smoothed by a smoothing circuit 134. The smoothing circuit 134 thus produces a DC voltage. This DC voltage is applied to the other input end of each of the differential amplifiers 125 and 130. The differential amplifiers 125 and 130 thus compare input voltages, respectively. Here, note that the analog switches 126 and 131 are interlocked. When the switches 126 and 131 are simultaneously turned on, differential voltages corresponding to DC components Y.sub.L and R, and Y.sub.L and B are applied to the voltage memories 132 and 127, respectively. The contents of the voltage memories 132 and 127 are thus changed from the initial values. The red signal R and the blue signal B respectively from the VCAs 111 and 115 are changed so as to match with the low-frequency luminance signal Y.sub.L. In other words, the gains of the VCAs 111 and 115 are changed, so that the differential voltages are eliminated, thus achieving a good white balance. Thereafter, the analog switches 126 and 131 are simultaneously released (OFF). Since the voltage memories 127 and 132 store the updated values of the differential amplifiers 125 and 130, respectively, the white balance is kept unchanged. The above operation of the white balance control requires a few seconds. Thereafter, the diffusion plate is removed from the optical path, and the light is incident on the pickup tube 104. When the color temperature of illumination during picture taking is changed, the analog switches are turned on and then off to obtain the proper white balance when a change in color temperature is small.
When a change in color temperature is great, another trimming filter must be used to achieve the optimal white balance. Assume that VCAs are used which have a wide change in the gain and a wide dynamic range. In this case, the optimal white balance can be obtained without using the trimming filter. Even if the amplitudes of the signals R, B and Y.sub.L are uniformly balanced, the S/N ratios between any two of these signals are not balanced. In the characteristics shown in FIG. 3, the S/N ratio of the red signal is degraded, thus degrading image quality. Furthermore, when the color temperature is changed, the spectral characteristics (sensitivity) of the low-frequency luminance signal Y.sub.L are changed, thus degrading the color reproduction except for the color temperature of 3,000.degree. K. In order to compensate for such degradation in image quality, a linear matrix circuit must be used which is changed in accordance with a change in color temperature, resulting in the complex circuit arrangement and an increase in the number of component parts. Furthermore, stable circuit operation cannot be performed. The use of a trimming filter may allow the elimination of the above drawbacks. However, if a great number of filters are used in a turret manner, the structure becomes complicated and bulky. Further, since such a structure is of a mechanical type, it has poor resistance to impact and vibrations. The trimming filter has a movable member which is subject to deterioration over time in the optical path, so that the movable member of the trimming filter cannot be dust-proofed. Even if the trimming filter can be fitted in front of the lens, ease in handling and carrying is decreased.